Modulation of a movable IR emitter through a diaphragm structure

ABSTRACT

The invention relates to a modulatable infrared emitter comprising an aperture structure, a structured micro-heating element, and an actuator, wherein the aperture structure and the structured micro-heating element are movable relative to each other in parallel planes by means of the actuator to modulate the intensity of emitted infrared radiation. The invention further relates to methods of manufacturing the infrared emitter, a method of modulating emission of infrared radiation using the infrared emitter, and preferred uses of the infrared emitter. In further aspects the invention relates to a system comprising the infrared emitter and a control device for regulating the actuator.

DESCRIPTION

The invention relates to a modulatable infrared emitter comprising anaperture structure, a structured micro-heating element, and an actuator,wherein the aperture structure and the structured micro-heating elementare movable relative to each other in parallel planes by means of theactuator to modulate the intensity of emitted infrared radiation. Theinvention further relates to methods of manufacturing the infraredemitter, methods of modulating emission of infrared red radiation usingthe infrared emitter, and preferred uses of the infrared emitter. Infurther aspects the invention relates to a system comprising theinfrared emitter and a control device for regulating the actuator.

BACKGROUND AND STATE OF THE ART

Modulatable infrared emitters (IR emitters) are relevant for a varietyof applications in spectroscopy. In particular, the spectroscopy ofgases is often performed with the aid of infrared radiation, which atcertain frequencies triggers vibrations of the molecules detectable asabsorption lines in the spectrum.

Photoacoustic spectroscopy is often used, employing intensity-modulatedinfrared radiation with frequencies in the absorption spectrum of amolecule to be detected in a gas. If this molecule is present in thebeam path, modulated absorption takes place, leading to heating andcooling processes whose time scales reflect the modulation frequency ofthe radiation. The heating and cooling processes lead to expansions andcontractions of the gas, causing acoustic waves at the modulationfrequency. Said acoustic waves can subsequently be measured by acousticdetectors (microphones) or flow sensors.

Photoacoustic spectroscopy allows the detection of very fineconcentrations of gases and has a variety of applications. An example isthe detection of CO₂, which takes an important role in research and airconditioning technology. The concentration of exhaust gases in the airmay for instance also be measured in this way. Military applications inwhich smallest concentrations of toxic gas can be detected may also berelevant.

Various emitters are used as radiation sources for the aboveapplications, with different advantages and disadvantages. For example,narrowband laser sources in the infrared range can be used. These allowthe use of high radiation intensities and can be high-frequencymodulated with standard components, e.g. for photoacoustic spectroscopy.However, due to the narrow spectrum of the laser, only molecules with amatching absorption spectrum can be detected. Lasers are also relativelyexpensive. If a detect of a number of different molecules is desired, acorresponding number of lasers must be used.

Thermal, broadband emitters are also known. These have the advantage ofa wide spectrum and often low cost. However, the modulation frequency ofthese emitters is limited, direct modulation by varying the currentsupply is slow due to thermal time constants and significantly degradesdevice life. Slow modulation often results in a measurement with a poorsignal-to-noise ratio due to the inherent noise of the detectioncomponents. External modulation through the use of spinning chopperwheels is faster, but the setup is costly and not as compact and robustas would be desirable for many applications. Also, modulation bandwidthsare limited and varying the rotation speed of the chopper is cumbersomedue to inertias.

Other approaches to modulating IR emitters are known in the prior art.

DE 195 26 352 A1 has set itself the task of improving the modulation ofthe beam path in nondispersive infrared gas analyzers compared to knownrotating aperture wheels. To this end, DE 195 26 352 A1 proposes torotate an emitter about an axis perpendicular to the beam axis and tomodulate the emitter by means of one or more apertures. In oneembodiment, a rod-shaped radiator at the focal point of a reflector isproposed, which is modulated in phase opposition by means of tworotating apertures offset by 90°. Thus, a fast rotation of the IRradiator itself, which is mounted on a shaft, is necessary to achieve ahigh-frequency modulation. This increases the construction effort andmakes a compact arrangement more difficult.

From GB 2502520 A an electro-optical simulation of light sources with atime-varying light intensity profile as defense signals is known. Ahomogeneous arc lamp with a broad spectrum in the infrared range is usedas the radiation source. To modulate the light intensity, the use of oneor more templates with a plurality of transmission ranges is proposed.In one embodiment, the stencil is tilted with respect to the lightsource to obscure the transparency regions as seen by the light source.Here, the necessary angle of inclination depends on the thickness of thestencil. In another embodiment, two stencils are shifted against eachother in order to cover the transmission areas from the point of view ofthe arc lamp. The modulation is intended to provide a simulation of alight source that allows a rapid rise and a gradual decay. Even in anopen position, a substantial amount of light is absorbed by thenon-transmission regions of the templates, reducing the maximum emissionamplitude.

U.S. Pat. No. 6,407,400 B1 relates to a modulation of infrared lightsources as a defense measure for heat-seeking missiles. The modulationmeans proposed are a collection element, a stator, and two rotorscomprising alternating opaque and translucent material. The constructionmethod of U.S. Pat. No. 6,407,400 B1 requires precise tuning of therotating elements. In addition, even in the case of U.S. Pat. No.6,407,400 B1, a substantial amount of infrared light is always absorbedby the non-transmissive during momentary open positions, reducing themaximum emission amplitude.

These approaches to providing modulated infrared emitters are thereforenot as compact, robust and energy-efficient as would be desirable formany applications. Today, microsystems technology is used to manufacturecompact mechanical-electronic devices in many fields of application. Themicrosystems (microelectromechanical systems, or MEMS) that can bemanufactured in this way are very compact (micrometer range) whileoffering outstanding functionality and ever lower manufacturing costs.For example, DE 10 2017 206 183 A1 describes fast and compact combdrives as MEMS actuators.

A use of MEMS technology to modulate a thermally generated infraredradiation is not known from the prior art.

OBJECTIVE OF THE INVENTION

It is an objective of the invention to provide a modulatable infraredemitter as well as a method for generating modulated infrared radiationwithout the disadvantages of the prior art. In particular, it was anobjective of the invention to provide a high-frequency and variablymodulatable infrared emitter which can emit a broad spectrum of infraredradiation in a modulated manner and at the same time is characterized bya simple, low-cost compact design.

SUMMARY OF THE INVENTION

The objective is solved by the features of the independent claims.Advantageous embodiments of the invention are described in the dependentclaims.

In a first aspect, the invention relates to a modulatable infraredemitter comprising.

-   -   an aperture structure,    -   a structured micro-heating element and    -   an actuator,    -   wherein the micro-heating element has heatable and non-heatable        regions in a first plane, the aperture structure has        transmissive (transparent) and non-transmissive (opaque) regions        for infrared radiation in a second plane, the two planes being        parallel to each other, the aperture structure and the        micro-heating element are movable in the parallel planes        relative to each other, and the actuator is configured for        relative movement of the aperture structure and the        micro-heating element between at least a first and a second        position, such that an extinction ratio of at least 2 is        achievable for the infrared radiation emittable from the        micro-heating element through the aperture structure between the        first and the second position. To this end the arrangement or        dimensioning of the heatable and non-heatable regions of the        structured micro-heating element and the arrangement or        dimensioning of the transmissive and non-transmissive regions r        of the aperture structure are preferably matched to one another        in such a way that, in the first position, the IR radiation        emittable by the heatable regions is predominantly absorbed        and/or reflected by the non-transmissive regions of the aperture        structure, while, in the second position, the IR radiation        emittable by the heatable regions predominantly radiates through        the transmissive regions of the aperture structure.

Due to the movability of the aperture structure relative to thestructured heating element, a modulation of the intensity of the emittedinfrared radiation can be achieved in a particularly fast and simplemanner. In contrast to known intensity modulations in infrared emittersby varying the current supply, the modulation according to the inventionis not limited by thermal time constants. Rather, MEMS actuators can beused to achieve modulation frequencies well above 100 Hz. Suchmodulation frequencies are particularly advantageous for photoacousticspectroscopy. However, the modulatable infrared emitter is suitable forany application where fast and reliable modulation of infrared radiationis required.

Primarily, the modulatable infrared emitter is a device that emitselectromagnetic radiation. This radiation preferably exhibits awavelength range in the infrared (IR) region, particularly between about700 nanometers (nm) and 1 millimeter (mm) wavelength. The correspondingfrequency of the emitted radiation may be in the range between about 300gigahertz (GHz) to 400 terahertz (THz). The spectrum may just aspreferably be represented in terms of the wavenumber m−1 or cm−1, as itis common in the field of spectroscopy. A person skilled in the artknows how to convert to these units. The term emitter preferably refersto the device comprising the radiation source, which is represented bythe micro-heating element, and an aperture structure, which enables themodulation of the infrared radiation by relative movements with respectto the micro-heating element.

In particular, the spectrum is selected to correspond to the preferredfield of application of the emitter, namely infrared spectroscopy andespecially photoacoustic spectroscopy. In particular, the vibrationalexcitation of the gas molecules to be analyzed and/or detected ispreferred, which correspond to a preferred spectral range depending onthe gas molecules. For example, a spectral range of the IR emitterencompassing a wavelength of about 2.4 micrometers (μm) is suitable forthe excitation of CO₂ molecules. Particularly preferred wavelengthranges of infrared radiation are 700 nm to 10 μm, preferably 1 to 5 μm,especially preferably 2 μm to 3 μm.

The radiation can be emitted isotropically, i.e. uniformly in allspatial directions starting from the emitter. In this context, uniformmeans preferably with the same intensity of the radiation. Intensity isdefined in particular as area power density and preferably has the unitwatts per square meter or abbreviated W/m². However, as opposed toisotropic emission of the radiation, it is preferred that the radiationis bundled in the form of a beam oriented along a preferred direction inthe form of a degree. Since the radiation of an emitter, especiallywithout additional components, typically diverges and can preferably bedescribed with respect to the emitting surface, e.g., by Lambert's law,additional components such as lenses can be integrated in the emitter orattached externally to provide for bundling or collimation of the beam.A person skilled in the art knows how to shape the emission profile ofthe radiation source by designing the radiation source as well as byusing additional components to result in a desired beam profile as wellas a desired beam direction. Preferably, the modulatable IR emitter maycomprise only the actual radiation source without additional lenses aswell as a system comprising radiation source and at least one lens forcollimation of the beam. In the further course, the term beam shalldescribe the preferably bundled part of the radiation along thepreferred beam direction of the emitter, which is emitted by theemitter, wherein particular the portions of greatest intensity alongsaid direction define the beam. For the radiation or beam propagatingbetween the micro-heating element and the aperture structure in thefollowing the terms unmodulated radiation or the unmodulated beam willbe used to allow for a distinction of the beam outside the IR emitter.

The emitter is modulatable, which means that the intensity of theemitted radiation, preferably the intensity of the beam can be changedin a controllable manner over time. The modulation shall preferablycause a temporal change of the intensity as a measurable quantity. Thismeans, for example, that there is a difference in intensity over timebetween the weakest intensity measured within the measurement period andthe strongest intensity measured within the same period that is greaterthan the sensitivity of an instrument typically used for the radiationspectrum and application to measure or determine intensity. Preferably,the difference is significantly greater than a factor of 2 between thestrongest and weakest adjustable intensities. A modulatable infraredemitter has a variety of applications. In terms of relevant applicationsinfrared spectroscopy and especially photoacoustic spectroscopy are tobe mentioned.

A thermal emitter in the form of a micro-heating element is provided togenerate the infrared radiation. A micro-heating element is preferablyunderstood to be a heating element with dimensions of the order ofmicrometers (μm). Here, the heating element comprises a heatable layerof a conductive material which produces joule heat when an electriccurrent flows through the material. The heat produced preferablyexhibits a dependence on the ohmic resistance of the element and thesquare of the current or the square of the applied voltage and theinverse ohmic resistance, depending on whether a current or voltagesource is used. In a state of equilibrium, the heat produced is equal tothe heat losses due to thermal conduction, convection and thermalradiation (synonymous: infrared radiation) emitted at the externalinterfaces of the heatable layer through which the current flows. As isknown to the person skilled in the art, the heat produced causes, i.a.thermal radiation, in particular by thermal movement of particles, whichresults, for example, in an acceleration of charge carriers and/oroscillating dipole moments. Thus, infrared radiation can be specificallygenerated by a current-carrying heatable layer. The heatable layer ispreferably made of metal, for example tungsten or platinum. By applyinga suitable voltage, the resulting current flow leads to the generationof joule heat and ultimately infrared radiation. The radiation spectrumcan preferably be described approximately by Planck's radiation law,wherein the person skilled in the art is aware of the differencesbetween an actual heatable layer and a black body, for example, theemissivity or the actual deviation from a thermal equilibrium of thebody. Despite these deviations, the generated spectrum and its intensityis essentially described by the temperature and the radiating areaaccording to Planck's radiation law. Thus, a skilled person can achievea preferred spectrum with a preferred intensity distribution by specificdesign of the micro-heating element. For this purpose, in addition tothe material and the geometric design of the heating element, theelectrical energy provided, a surface treatment of the radiatinginterface, and the magnitude of the heat losses of the heating elementin addition to the thermal radiation are preferably decisive. Themagnitude of these heat losses is determined, for example, by thethermal conductivity between the heating element and the adjacentmaterials and/or fluids as well as their heat capacity and the size ofthe interface(s).

The structured micro-heating element is preferably characterized by atwo-dimensional plane, the first plane, in which, heatable andnon-heatable regions are present. Heatable regions are regionscomprising a heatable layer of a conductive material as described above.A non-heatable region is preferably defined by not being a heatableregion and being adjacent to a heatable region or between two heatableregions. When a current is applied, infrared radiation is preferablyemitted from the heatable regions in the direction of emission, whilethis is not the case for the non-heatable regions.

Preferably, the heatable and non-heatable regions within the first planeare arranged substantially along a line. For example, the micro-heatingelement could comprise a surface of a cuboid substrate that constitutesthe first plane. On this surface, in the form of electrically contactedstrips (e.g., coatings), the heatable regions may be deposited. Thesecould be oriented, for example, perpendicular to the long side of thecuboid surface. Between these strips, there may be essentiallystrip-shaped, non-heatable regions. It may be preferred that theheatable strips are shorter than the short side of the cuboid surface,so that all non-heatable regions are connected along one long side ofthe cuboid surface. Even though the non-heatable regions form aconnected surface in this embodiment, there are multiple non-heatableregions for the purposes of the invention. In particular, heatable andnon-heatable regions alternate along the centerline of the cuboid,which, relative to an appropriately selected aperture structure, allowsfor the modulation according to the invention.

For example, the heatable regions may be in the form of a coating of thesubstrate having a thickness that is small compared to the extent withinthe first plane. However, it may also be that the heatable regions havea significantly greater thickness. However, even in this case, therelevant surface for the purposes disclosed herein is the one at whichthe emission of the infrared radiation is essentially generated and canbe described by a normal in the direction of emission. Said surfaceforms the first plane. A normal to the first plane thus preferablyindicates the emission direction in which the emitted intensity of theinfrared radiation is strongest compared to other directions and/orwhich relates to the preferred direction of emission. Said first planepreferably simultaneously forms an (intersection) plane with theheatable and non-heatable regions.

The micro-heating element is preferably at least partially free-standingand allows, for example, thermal expansion within the IR emitter due tostrong temperature changes as well as translational movements. Partiallyfree-standing means that it is at least partially non-positively and/orpositively connected to other elements of the emitter at the interfacesand therefore has a degree of freedom of movement in a directionessentially perpendicular to the interface.

Modulation of the intensity emitted by the infrared emitter (IR emitter)can be achieved by controlled and repeatable temporary blocking of theunmodulated beam by an element that is non-transmissive (opaque) forsaid beam. To this end, the aperture structure comprises transmissive(transparent) and non-transmissive (opaque) regions for infraredradiation. The aperture structure is preferably characterized in that itcomprises transmissive and non-transmissive regions for infraredradiation within a plane parallel to the first plane of the heatingelement (second plane). The regions are preferably arranged within thesecond plane along a line.

The aperture (blend) structure is preferably a flat element which, withthe exception of the regions transmissive (transmissive) to infraredradiation, consists of a material that is non-transmissive (opaque) toIR radiation. The transmissive regions can be formed, for example, byin-plane slots in the aperture structure. Similarly, in these regions adifferent material that is substantially transparent to the spectrum ofradiation may be used. It may be equally preferred that the material(for forming the aperture) is substantially transparent to infraredradiation and the non-transmissive regions are formed, for example, by acoating substantially opaque to infrared radiation.

The non-transmissive regions of the aperture structure are of a materialsubstantially opaque to infrared radiation. It is preferred that thismaterial, when blocking infrared radiation therefrom, is not heated tosuch an extent that it itself begins to emit infrared radiation at alevel that is contrary to the desired modulation characteristics. Thus,it may be desirable that the material substantially reflects theradiation and/or that any heat generated by absorption of the IR beam besufficiently dissipated.

The arrangement or dimensioning of the transmissive and non-transmissiveregions of the aperture structure is preferably matched to thearrangement or dimensioning of the heatable and non-heatable regions ofthe structured micro-heating element in such a way that, in the firstposition, the IR radiation emittable by the heatable regions ispredominantly absorbed and/or reflected by the non-transmissive regionsof the aperture structure, while in the second position the IR radiationemittable by the heatable regions predominantly radiates through thetransmissive regions of the aperture structure. By suitable selection ofthe size as well as arrangement of the areas of the transmissive andnon-transmissive regions of the aperture structure or of the heatableand non-heatable regions of the structured micro-heating element, it canthus be achieved, for example, that in a first position the IR radiationemittable from the heatable regions is almost completely absorbed by thenon-transmissive regions of the aperture structure, while in a secondposition the IR radiation emittable from the heatable regions radiatesalmost completely through the transmissive regions of the aperturestructure. By aligning the structuring of the micro-heating element andthe aperture structure, a particularly high modulation depth or highextinction ratios can be thus achieved with simple means.

As an example the aperture structure may be essentially planar andrectangular and exhibit slit-shaped transmissive regions. Thesetransmissive regions could run perpendicular to the long side of therectangle, for example. Between the transmissive slits are essentiallystrip-shaped non-transmissive regions. It may be preferred that thetransmissive regions are shorter than the transverse side of therectangular aperture structure, so that all non-transmissive regions areconnected to each other along at least one longitudinal side of theaperture structure. Even though in this case the non-transmissiveregions geometrically form a connected surface, in the sense of theinvention several non-heatable regions are present. In particular,transmissive and non-transmissive regions for IR radiation alternatealong the center line of the cuboid, which allows for the modulationaccording to the invention with respect to a correspondingly structuredheating element.

Preferably, a substantially free-standing, self-supporting aperturestructure can be provided. In particular, the regions of the aperturestructure that are transmissive (transparent) and non-transmissive(opaque) to infrared radiation are intended to transmit or block theunmodulated beam emitted by the micro-heating element depending on thepositioning of the aperture structure within the second plane.Therefore, the aperture structure and micro-heating element shall bemovable relative to each other such that the unmodulated beam issubstantially blocked by the non-transmissive regions in at least afirst (relative) position such that the intensity of the beam (on theside of the aperture structure facing away from the heating element)becomes minimal, and is substantially transmitted through thetransmissive regions in at least a second (relative) position such thatthe intensity of the IR beam becomes maximal. Herein, the relativemotion should take place between the two parallel planes. Preferably,the actual movement may be performed by the aperture structure in itsplane and/or by the micro-heating element in its plane. The movementpreferably takes place within one of the two planes along a preferreddirection. The preferred direction can in particular be defined by thedirection along which the regions of the micro-heating element and/orthe aperture structure are arranged. When arranging the corresponding(non-)transmissive and (non-)heatable regions along a line, a linearrelative movement is preferably intended. If the regions are arranged ona circle, a rotational movement may be preferred.

A person skilled in the art is familiar with the design and operation ofvarious suitable actuators, in particular MEMS actuators for both lineartranslational and rotational movements, or can refer to the relevanttechnical literature (see, among others, Judy J. W. (2006)Microactuators. In: Korvink J. G., Paul O. (eds) MEMS: A Practical Guideto Design, Analysis, and Applications. Springer, Berlin, Heidelberg, E.Thielick, E. Obermeier Microactuators and their technologiesMechatronics Vol. 10, 4-5, 1 Jun. 2000, Pages 431-455, Elwenspoek, M.,Wiegerink, R. J., Mechanical Microsensors, Springer, Berlin, Germany,2001, M. Tabib-Azar Microactuators, Springer Science+Business Media NewYork 1998).

Terms such as substantially, approximately, about, etc. preferablydescribe a tolerance range of less than ±40%, preferably less than ±20%,particularly preferably less than ±10%, even more preferably less than±5%, and especially less than ±1%. Similar preferably describes sizesthat are approximately equal. Partially preferably describes at least5%, more preferably at least 10%, and more preferably at least 20%, insome cases at least 40%. For example, if it is disclosed in theforegoing that a region is substantially transparent to an infraredbeam, it is meant that the entire intensity of a beam or partial beam istransmitted through said region within the above tolerance ranges.

The aperture structure and the micro-heating element are movable in theparallel planes to each other. If the modulatable IR emitter is providedwith a housing, the aperture structure and/or the micro-heating elementis preferably movably mounted relative to said housing. Therefore, themovably mounted element may be connected to the rigid elements via alinear guide. A linear guide preferably allows linear movement along onedirection and prevents movement or restricts the degree of freedom ofmovement in other directions. At the same time, a linear guidepreferably allows movement along one direction with as little frictionand maintenance as possible, for example by means of rolling elementsand/or plain bearings.

The actuator is configured for a relative movement of the aperturestructure and the micro-heating element. In particular, an actuatorconverts an electrical control signal into a movement. The actuator canbe a MEMS actuator, which is for example an electrostatic actuator. Theactuator can be directly connected to the movable aperture structureand/or the movable micro-heating element. In particular, it is preferredthat the actuator is at the same time a connecting element (joint) ofthe movable element with the rigid part of the IR emitter, in particularthe housing of the emitter. Thus, in particular, the actuator canfurthermore be a linear guide at the same time. It is particularlypreferred that the actuator is the only connecting link between themoving structure and the rest of the emitter. The moving structure canotherwise be essentially free-standing. In this way, a particularlysimple and compact structure of the IR emitter may be achieved. It isparticularly preferred that the micro-heating element is moved relativeto the aperture structure by an actuator. Thus, by moving the heatableregions of the micro-heating element relative to the regions of theaperture structure that are transparent to IR radiation, a modulation ofthe IR beam can be performed by substantially blocking the IR radiationin at least one first position and transmitting it substantiallycompletely through the transmissive regions in at least one secondposition, so that any intensity in the range between the minimum andmaximum intensities reached in these two positions can be set in adesired time course. Thus, the first position preferably corresponds toa minimum intensity and the second position preferably corresponds to amaximum intensity.

The ratio between the maximum and minimum intensity of the emitted IRradiation, which can be adjusted by the relative movement, is referredto as an extinction ratio. The extinction ratio can be determineddirectly from the quotient between maximum intensity and minimumintensity, and preferably may be specified directly by said quotient.However, it may also be preferred that the ratio is expressed in thelogarithmic scale decibel (dB), as is common in communicationsengineering, for example.

The actuator is preferably configured for the relative movement if itcan perform the relative movement over the entire range at least betweena first position and a second position at a modulation frequencysuitable for a desired modulation frequency and can be driven by anelectrical signal according to the requirements.

It may also be preferred that the aperture structure is moved relativeto the heating element by an actuator. The modulation described abovemay be achieved in the same manner.

If the micro-heating element is characterized by several heatableregions, for example in strip form, the emitted beam of the IR emitteris characterized by the combined partial beams of the individual areasand their intensity. The exact geometric radiation behavior of theindividual areas is thereby preferably dependent, i.a. on the overalldesign of the IR emitter, for example on the geometric design of theheatable regions, the distance from the aperture structure to themicro-heating element, the positioning of a lens for collimating thebeam, etc.

For example, a lens can be placed between the heating element and theaperture structure, but it can also be placed on the side of theaperture structure facing away from the heating element. Moreover, adistance between the heating element and the aperture structure may besmall enough such that the aperture structure is in the near field.Likewise, the distance may be greater so that the radiation at theaperture structure is described by the far field. Independent of this,it is preferred that in a first position within the relative motionbetween the aperture structure the beam is substantially blocked by theaperture structure and in another, second position the radiation or beamis substantially transmitted by the aperture structure. To achieve thedesired extinction ratio during modulation, it is decisive that theratio between the minimum and maximum intensity of the IR emitter isappropriate. Therefore, it may also be preferred that the beam is onlypartially transmitted even in the second position of maximum intensityas long as the beam is substantially blocked in the first position ofminimum intensity, thus achieving a desired extinction ratio.Preferably, the intensity of the beam to be considered relates to themodulated intensity of the beam after it leaving the IR emitter andbeing available for further use. Preferably, the minimum and maximumintensities denote the spatially averaged intensities in the emissiondirection directly after the aperture structure.

It is preferred that the aperture structure is designed to the structureof the micro-heating element. In particular, this means that the regionstransparent to IR radiation match the heatable regions of the heatingelement in shape, number and spacing, taking into account the divergenceof the unmodulated radiation emitted by the heating element. In thismanner, the desired modulation behavior may preferably be achievedindividually for each partial beam emitted by a heatable region, andthus the total beam composed of the partial beams may equally bemodulated as desired. For example, in the case of a micro-heatingelement comprising a plurality of parallel, strip-shaped heatableregions, it may also be preferred to use an equal number of parallel,strip-shaped transmissive regions of the aperture structure. A personskilled in the art would know how to design the IR emitter with respectto the aperture structure, the micro-heating element, the spacing of thetwo components, etc., to obtain the desired modulation characteristics.The person skilled in the art would know, for example, that he may haveto select the non-transmissive strip-shaped regions of the aperturestructure between transmissive strip-shaped regions wider than theheatable strip-shaped regions of the micro-heating element in order toaccount for the divergence of the emitted radiation and to block saidradiation to a sufficient extent.

It is particularly preferred that an extinction ratio of at least 2 isachievable for the infrared radiation emitted by the micro-heatingelement through the aperture structure between the first and secondpositions. Said ratio is preferably the direct quotient between themaximum and minimum intensity. However, it may also be preferred toselect the structuring to allow higher extinction ratios of, forexample, at least 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 100,200, 300, 400, 500 or 1000. The extinction ratio can likewise beexpressed in dB, with extinction ratios of at least 3 dB, at least 10dB, at least 20 dB, at least 30 dB, or at least 40 dB being preferred.The preferred extinction ratios allow for a particular well realizationof the desired applications, e.g. in photoacoustic spectroscopy.

The maximum modulation frequencies achievable at the desired modulationdepths should preferably be at least 1 kilo Hertz (kHz), particularlypreferably at least 10 kHz, more preferably at least 20 kHz,particularly preferably at least 30 kHz, and especially at least 100kHz. It is particularly preferred to achieve modulation frequencies inthe range of audible sound and/or ultrasound for an application inphotoacoustic spectroscopy. The modulation bandwidth over which thedesired modulation depth is achieved preferably concerns the entirefrequency range from 0 Hz to the maximum modulation frequency.

It may also be preferred that there are more positions than only a firstand/or a second position where the intensity becomes minimum and/ormaximum. It may be equally preferred that the positions relate only tolocal intensity maxima and/or minima, which however also fulfill thedesired extinction ratio in conjunction with another position.

The desired modulation can preferably find expression in a correspondingtemporal course of the emitted radiation intensity. To determine thefeasibility of such a desired temporal intensity course, the modulationdepth and the bandwidth over which this modulation depth is essentiallyachievable are of particular importance. Moreover, the resolution of anelectronic control of the IR emitter is preferably relevant forfeasibility. For example, it may be of importance which differentintensity levels between minimum and maximum intensity can be achievedwith which frequency. It is preferred that the IR emitter exhibits anelectrical control that controls the micro-heating element and therelative movement between the heating element and the aperturestructure.

Such a control can be realized e.g. by a control device. By means of acontrol device, the desired spectra, intensities and modulations may beset or adjusted. Control preferably means that electrical controlsignals are transmitted directly to the actuator and the micro-heatingelement, which result in the desired radiation properties. In the caseof the micro-heating element, this means that in particular a specifictemperature and/or a specific temporal temperature profile may be set oradjusted. Furthermore, a certain modulation signal can be obtained bythe relative movement triggered by the actuator (possibly incoordination with a temperature course). Typically, the modulationsignal is an analog signal generated by a control device. This in turncan preferably receive a suitable digital electronic signal, for examplefrom a control computer, which is subsequently translated by the controldevice into suitable actuation signals.

It is particularly preferred a number of parts of the IR emitter, suchas the micro-heating element and actuator are MEMS elements that exhibitsmall dimensions in the micrometer range and are produced according tostandard manufacturing processes.

The structure of the modulatable infrared emitter will now beillustrated by means of a specific embodiment. The IR emitter ispreferably accommodated in a housing consisting of a lower support, sideparts and a cover element. Sealing elements can be provided between thecarrier, the cover element and the side parts. The sealing elements maybe used to reduce a thermal exchange of the micro-heating element housedinside the housing with the external environment of the housing. Thecover element exhibit an applied aperture structure. The structuredmicro-heating element within the housing comprises individual, parallelheating lamellas whose surfaces oriented in the direction of theaperture structure represent heatable regions in a first plane. Alongsaid first plane, non-heatable regions are located between the heatableregions. The regions are each arranged periodically. The aperturestructure is arranged along a second plane parallel to the first planeand consists of regions that are transmissive (transparent) to infraredradiation and regions that are non-transmissive (opaque).

The relative movement is achieved by an actuator in the form of a combdrive, which is directly coupled to the micro-heating element. Theactuator is in turn attached to a side part of the housing. Themicro-heating element is only connected to the housing via the actuatorand is otherwise free-standing.

In the exemplary embodiment described, the number of non-transmissiveregions of the aperture structure is equal to the number of heatableregions of the micro-heating element. The regions are moreover arrangedperiodically. The width of the non-transmissive regions is slightlywider than that of the heatable regions, so that their IR radiation issubstantially blocked when the heatable regions are positioned by meansof the actuator directly below the non-transmissive regions in the firstposition in which the radiation emitted from the IR emitter exhibits aminimum intensity. By moving the heatable regions to a second positionbelow the transmissive regions of the aperture structure, a maximumintensity of the emitted beam may be set. In this case, the regions areconfigured relative to each other such that an extinction ratio betweenthe intensity of the radiation emitted in the second position and theintensity of the radiation emitted in the first position of at least 2is achieved.

Such an IR emitter achieves a number of preferred characteristics, it isfast modulating, the modulation depth (extinction ratio) is suitable formany applications, it is compact, robust and durable. The bandwidth ofthe modulation is furthermore greatly improved compared to themodulation methods known from the prior art.

In a preferred embodiment of the modulatable infrared emitter, in thefirst position, IR radiation emittable from the heatable regions ispredominantly absorbed and/or reflected by the non-transmissive (opaque)regions of the aperture structure, while in the second position, IRradiation emittable from the heatable regions predominantly traverses(passes through) the transmissive regions of the aperture structure.

The embodiment is a preferred embodiment of the modulatable infraredemitter due to a relative movement of the aperture structure and themicro-heating element between a first position and a second position. Inthis regard, the aperture structure and the micro-heating element arepreferably geometrically aligned such that the heatable regions in thefirst plane and the non-transmissive regions in the second plane may bepositioned on top of each other along a direction orthogonal to saidplanes.

Preferably, the non-transmissive regions completely cover the heatableregions and are positioned above the heatable regions in the directionof emission of the IR radiation. Completely covering means in particularthat the non-transmissive regions have an extension in each directionwithin the second plane which is at least equal to, and particularlypreferably greater than, the heatable regions in the first plane. It ispreferred that each heatable region is assigned an opaque(non-transmissive) region in this way. However, a non-transmissiveregion can also be assigned to several heatable regions. Of importanceis that the IR radiation emitted by the heatable regions ispredominantly absorbed and/or reflected, i.e. above all is nottransmitted, by the non-transmissive regions of the aperture structure.

It may be preferred that the non-transmissive regions substantiallyreflect, rather than absorb, the unmodulated radiation to avoid heatingthe aperture structure.

In a further preferred embodiment of the modulatable infrared emitter,the actuator is coupled to the heating element and configured fortranslational movement of the heating element relative to the aperturestructure. In this embodiment, the aperture structure is preferablystationary, wherein a relative movement between the aperture structureand the micro-heating element results from a translational movement ofthe heating element, and the movement is initiated by the actuator. Atranslational movement refers in particular to a displacement of theheating element. The displacement is preferably to occur within thefirst plane. Coupled means in particular that there is a directmechanical connection between the micro-heating element and the at leastone movable element of the actuator, so that a movement of the movableactuator element triggers a movement of the heating element in thedesired direction.

The actuator and heating element can preferably be directly connected toeach other. It may even be preferred that both heating element andactuator comprise the same substrate and/or are made of the samesubstrate. There may be not only a mechanical, but also a thermal and/orelectrical coupling to the actuator. Through thermal coupling, a desirednon-radiative heat loss of the heating element can be achieved, whichinfluences the radiative and/or modulation properties of the heatingelement in a desired manner. Electrical coupling may achieve electricalcontacting of the heatable layer of conductive material of the heatingelement. Advantageously, if the heating element is movable, the aperturestructure can be installed stationary, e.g. in a cover element of thehousing, which increases the robustness of the emitter.

However, it may be equally preferred that the actuator is coupled to theaperture structure and configured for translational movement of theaperture structure relative to the heating element.

In the embodiment, the micro-heating element is preferably stationary,wherein the relative movement between the micro-heating element and theaperture structure is achieved by a translational movement of theaperture structure, and the movement is triggered by the actuator. Atranslational movement in this case preferably denotes a displacement ofthe aperture structure. This is preferably to occur within the secondplane. In this embodiment, coupled means in particular that there is adirect mechanical connection between the aperture structure and the atleast one movable element of the actuator, so that a movement of themovable actuator element triggers a movement of the heating element inthe desired direction.

Actuator and aperture structure can preferably be directly connected toeach other. It may even be preferred that both the aperture structureand the actuator comprise the same substrate and/or are made of the samesubstrate. There may be not only a mechanical but also a thermalcoupling between the actuator and the aperture structure. The thermalcoupling can be used to dissipate heating of the aperture structure byany absorbed radiation. However, it may also be desirable that theaperture structure and the actuator are thermally decoupled to prevent aheat transfer from the heating element to the structure via theactuator.

In a further preferred embodiment of the modulatable infrared emitter,the infrared emitter comprises a housing in which the aperturestructure, the micro-heating element and the actuator are presentinstalled. Here, it is particularly preferred that the aperturestructure is thermally decoupled from the housing.

Preferably, the housing may be based on the dimensions and shapes of theinstalled components, it may be equally preferred that the housing issignificantly larger than the installed elements to improve handling ofthe emitter and create a robust device. For example, the micro-heatingelement, actuator and/or aperture structure may be MEMS elements and/orhave dimensions in the (sub-) micron range, with the housing havingdimensions in the centimeter range.

It is possible, as already described above, that the actuator within thehousing is directly coupled to the micro-heating element/aperturestructure. Moreover, a general structure of the emitter, comprising ahousing has already been described in an exemplary embodiment above.

Preferably, the housing exhibits a continuous outer surface and isclosed on the inside. The micro-heating element is installed inside thehousing. This allows it to be protected from external influences andprevents emission of IR radiation to the outside except through thetransmissive regions of the aperture structure in appropriatepositioning. The actuator can preferably be attached to a side part ofthe housing.

It is preferred that the micro-heating element is not thermally isolatedfrom the housing, but rather that non-radiative heat dissipation fromthe heating element to the housing is possible so that heat candissipate from the heating element. For example, a desired balance canbe established between the heat generated by the current-carrying,heatable layer of conductive material and the heat dissipated from theheating element to the environment, the desired radiationcharacteristics can be produced, and/or the desired modulationcharacteristics can be achieved.

It may be preferred, for example, that the components housing,micro-heating element and/or actuator are made of the same material andthat there is sufficient thermal conduction between directly connectedelements.

The housing may preferably comprise a heat sink for its own heatdissipation.

It is desirable that the aperture structure, which is also presentinstalled in the housing, is thermally decoupled from the otherelements, in particular from the housing. This preferably means that byusing at least one suitable material at the connection between theaperture structure and the housing or actuator and/or by a suitabledesign of the connection point (for example small connection area and/orsuitable thickness of the connection) the aperture structure does notheat up significantly. Heating up is preferably described in relation toa temperature of the aperture structure when the micro-heating elementis switched off and in thermal equilibrium.

Likewise, it may be preferred that the time constant essential fordetermining the time course of the approximation of the temperature ofthe aperture structure to the housing is sufficiently large. This can,for example, be greater than 1 minute, preferably greater than 10minutes, and in particular greater than one hour.

A suitable material at the joint preferably covers the entire jointsurface. Suitable materials refer in particular to the thermalconductivity of the materials, expressed in watts per meter and kelvin(W/m·K). Preferred thermal conductivities at the junction are less than10 W/m·K, particularly preferably less than 1 W/m·K and especially lessthan 0.1 W/m·K.

Preferably, an oxide layer is introduced at the connection point betweenthe aperture structure and the housing or actuator and/or between theactuator connected to the aperture structure and the housing to providethe desired thermal decoupling. An oxide layer is particularly wellsuited to providing thermal decoupling in the materials used for theaperture structure. Moreover, they are particularly easy and inexpensiveto produce.

In order to minimize the direct transfer of heat between themicro-heating element and the aperture structure, it may be preferredthat the housing is configured for generation of a vacuum in a spacebetween these components or between the first and second planes. Avacuum preferably refers to a pressure of less than 30×10³ Pascals (Pa),more preferably less than 100 Pa, and more preferably 0.1 Pa or less.Configured means that the housing is designed to be sufficientlypressure tight. It is also preferred that the housing comprises aconnection for a vacuum pump or an integrated vacuum pump. However, itis equally preferred that the housing is substantially permanentlyevacuated during manufacture.

It may also be preferred that the aperture structure is cooled tominimize its own emission of unmodulated IR radiation in the directionof the modulated beam. For example, Peltier elements and/or fluidcooling may be used to this end.

In a further preferred embodiment of the modulatable infrared emitter,the housing comprises a cover element in which the aperture structure ispresently fixated and in which at least one optical filter isadditionally installed in the cover element. The aperture structure isthus preferably integrated into the housing and is present on one outersurface of the housing, which is formed by the cover element. Theaperture structure may essentially form the cover element or becomprised by the cover element.

Depending on the use of the IR emitter, e.g. in various spectroscopymethods, either the entire broad frequency spectrum of the thermalradiation source may be used or narrower spectra are desired. To selecta desired spectrum, which differs significantly from the unmodulatedspectrum of the micro-heating element, frequency filters can preferablybe employed. Advantageously, these may be integrated into the coverelement.

Filters may be positioned between the micro heater and the aperturestructure as well as on the other side of the aperture structure.

The filters used can advantageously exhibit different filtercharacteristics, e.g. band-pass filters, short-pass filters, long-passfilters, notch filters and any combination of these filters that lead tothe desired spectral influences may be used. The frequencies orfrequency ranges in which the filters act can be chosen flexiblydepending on the application.

For example, a filter wheel can be used as a filter, on which filterswith different filter properties are installed. The desired filter canbe selected mechanically by rotating the filter wheel. Preferably, thefilter wheel can be rotated by an electric drive.

The use of a Fabry-Perot filter is also conceivable. Such a filter canbe used, for example, to select very narrow spectra. Preferably, theFabry-Perot interferometer on which the filter is based tunable, forexample by tuning the temperature or by mechanical adjustment. Thus,desired spectra can be flexibly selected from the original beam.

Likewise, suitable thin-film filters can preferably be used. These areparticularly easy to manufacture and are very compact. In particular, ifthe IR emitter is manufactured in an integrated design in amanufacturing process, the production of such a thin-film filter can beeasily integrated into the process. This reduces costs.

A flexible combination of thin-film filters or employment of a thin-filmfilter tunable for example by changing the temperature is alsoadvantageous.

Filters for other properties of the IR radiation, e.g. polarization, canalso be used. Preferably they may also be part of the cover element.

In a preferred embodiment of the modulatable infrared emitter, themicro-heating element comprises a substrate on which is deposited, atleast in part, a heatable layer of a conductive material on whichcontacts for a current and/or voltage source are present.

The substrate preferably forms the base of the micro-heating element. Inthis context, the substrate may also comprise other components of the IRemitter, such as the actuator and/or housing elements, at least in part.Advantageously, the substrate can be suitably formed by establishedprocess steps, in particular from semiconductor and/or microsystemmanufacturing. Subsequently, preferably, a heatable layer of aconductive material can be applied to or integrated into the substrate,e.g., by doping and/or coating. The heatable layer preferably comprisesthe heatable regions of the micro-heating element. It is preferred thatthe heatable layer is contacted to a source of electrical energy toestablish electrical contact. Primarily, the contacting is to beperformed such that the heatable regions are at least partiallytraversed by electric current and emit IR radiation in a desired manner.

In a further preferred embodiment of the modulatable infrared emitter,the substrate is selected from a group comprising silicon,monocrystalline silicon, polysilicon, silicon dioxide, silicon carbide,silicon germanium, silicon nitride, nitride, germanium, carbon, galliumarsenide, gallium nitride and/or indium phosphide. These materials areparticularly easy and inexpensive to process in semiconductor and/ormicrosystem manufacturing and are also well suited for mass production.Likewise, these materials are particularly suitable for doping and/orcoating in order to achieve the desired electrical, thermal and/orradiation properties in certain regions.

In a further preferred embodiment of the modulatable infrared emitter,the conductive material for forming the heatable layer is selected fromthe group comprising platinum, tungsten, (doped) tin oxide,monocrystalline silicon, polysilicon, molybdenum, titanium, tantalum,titanium-tungsten alloy, metal silicide, aluminum, graphite and/orcopper. On the one hand, these materials exhibit the desired thermal,electrical, mechanical and/or radiation properties, and on the otherhand, they are particularly easy and inexpensive to process.

In a further preferred embodiment of the modulatable infrared emitter,the micro-heating element comprises a lamellar structure, a meanderstructure, and/or a grid structure.

A lamellar structure preferably refers to an arrangement of similarlayers running in parallel. The heatable layers of conductive materialare preferably arranged in lamellar form. The individual layers, alsoreferred to as lamellae in the following, are preferably arranged withtheir surface perpendicular to the first plane.

It may be preferred that the lamellae are planar, which means inparticular that their extension in each of the two dimensions of asurface is greater than in a dimension of the thickness perpendicularthereto. For example, the size ratios can be at least 1.5 to 1.Significantly larger ratios of, for example, 5 to 1 or 10 to 1 are alsocovered herein. The intersecting surfaces of the lamellae with the firstplane, or the side surfaces of the lamellae lying in the first plane,preferably form the heatable regions of the micro-heating element.

It is preferred that between the lamellae regions are present, which maybe parts of the substrate which do not comprise heatable layers ofconductive material. The intersection of these regions with the firstplane preferably form the non-heatable regions. The regions between thelamellae are preferably adapted for thermal expansion of the lamellae.

It is preferred that the heatable lamellas are electrically contacted toeach other via the substrate situated between them, thus exhibiting acommon contact to an electrical energy source.

Lamellae are particularly easy and inexpensive to produce on a substrateand are especially well suited for use as heatable regions.

A meander structure preferably denotes a structure comprising a sequenceof mutually orthogonal sections. Such a meander structure can, forexample, be formed from the above lamellar structure if adjacentlamellas are connected to one another at a side face. The meanderstructure is preferably formed by a heatable layer of conductivematerial.

A meander structure can be arranged in different ways within the IRemitter. Preferably, the first plane or a plane parallel to it can beused as a defining plane of the structure, e.g. as a symmetry plane. Theintersecting surfaces and/or boundary surfaces of the meander structurewith the first plane preferably form the heatable regions. It ispreferred to arrange the meander structure in such a way that the areaof the heatable regions is maximized.

Preferably, between substantially adjacent parallel connected orthogonalsections of the meander structure a substrate is present situated thatdoes not include a heatable layer of conductive material.

Such a meander structure can convey advantages during manufacture. Inparticular, such a structure inherently provides electrical contact forall heatable layers of conductive material or all heatable regions.

A meander structure, by virtue of being a continuous heatable layer ofconductive material, can exhibit a high resistivity and thus beparticularly efficient in producing a temperature distribution desiredfor IR radiation.

A grid (or mesh, lattice, grating) primarily refers to a periodicallyarranged structure. This structure is preferably formed by at least oneheatable layer of conductive material. Preferably, the structure hasinterfaces and/or intersections with the first plane representing theheatable regions of the micro-heating element. Preferably, the periodicstructure is disposed along the first plane and maximizes the area ofthe heatable regions. Preferably, non-heatable regions are locatedbetween adjacent heatable regions.

Such a grid provides great flexibility for the structuring of themicro-heating element.

In a further preferred embodiment of the modulatable infrared emitter,with respect to the possible relative movement between the micro-heatingelement and the aperture structure, in the first plane the heatable andnon-heatable regions of the micro-heating element and in the secondplane the transmissive and non-transmissive regions of the aperturestructure are periodically arranged.

The periodicity preferably denotes a repeating spatial distance of theregions within the first or second plane. The distance is preferablymeasured for the regions between two equally arranged points withinadjacent regions. An example of such a periodic arrangement arestrip-shaped heatable and/or transmissive regions which have the samedistance from each other in the transverse direction. It is furtherpreferred that the respective strips have the same dimensions. However,the latter is not necessary for the periodicity of the strips. Strips ofdifferent widths may also exhibit periodicity, which becomes apparent,for example, when the distance between adjacent regions is measuredbetween the perpendicular bisector of the transverse sides. Thenon-heatable regions and/or non-transmissive regions, which are situatedbetween the complementary, heatable regions and the transmissiveregions, respectively, and are also essentially strip-shaped, typicallylikewise exhibit a periodicity due to the periodicity of thecomplementary areas, which can be determined between at least two pointsof these regions.

By providing a periodic structure, a desired beam and/or modulationbehavior can be achieved in a particularly simple manner. Forillustration imagine a single strip-shaped, heatable region. In thiscase, the aperture structure comprises two strip-shaped, transmissiveregions, between which there is a non-transmissive (opaque) strip-shapedregion. Now, the micro-heating element is to be translated back andforth at a predetermined frequency f for modulation. Here, a translationperiod includes the following movement: from the position of theheatable region under the non-transmissive region to a position underone transmissive region then to the other transmissive region and backto the starting point. Thus, within one translational movement, twopositions are reached where the transmission becomes maximum (secondposition) and twice the same first position where the transmissionbecomes minimum.

Thus, in this example, the IR emitter may achieve an approximateaveraged modulation frequency of the IR beam of 2·f at a translationfrequency of f. In calculating the modulation frequency, it ispreferable to note that the translational motion at and near thereversal points of the reciprocating motion has a different velocitythan in the middle between two reversal points. Thus, the modulationfrequency is typically only approximately a multiple of the translationfrequency.

A variety of combinations of periodically arranged, (non-) heatableregions and/or (non-) transmissive regions are conceivable in order toproduce a wide range of desired modulation and/or beam properties. Forexample, constellations may be selected in which a modulation frequencyof approximately x·f can be achieved, where f is the frequency of thetranslational motion and x is an integer, preferably determined by thenumber of second and first positions passed.

The above-mentioned, lamellar, meander and/or grid structures areparticularly suitable for periodically structured micro-heatingelements.

In a further preferred embodiment of the modulatable infrared emitter,the spatial period of the arrangement of the heatable and non-heatableregions of the micro-heating element is equal to the spatial period ofthe arrangement of the transmissive and non-transmissive regions.

By matching the periods of both regions in such a manner, furtheradvantages may be achieved. For example, it may be preferred that aplurality of periodically arranged heatable regions are used to generatethe modulated beam to obtain a desired intensity and/or beam profile. Ifperiodically equally arranged (non-) transmissive regions of theaperture structure are used for this purpose, modulation with a desiredextinction ratio and frequency can be achieved even with very smalltranslational movements. As a consequence, a very efficient andminiaturized IR emitter with a large modulation bandwidth may beprovided. In particular, smaller translational motions can often beperformed at higher speed and/or frequency and/or by using MEMSactuators.

Preferably, the number of heatable regions is equal to the number ofnon-transmissive regions.

Furthermore, it is preferred that the translational motion is performedsuch that the same number of heatable regions is always below atransmissive region in every second position so that the intensity isunchanged between different second positions.

It may also be preferred that there are more heatable regions thantransmissive regions, or more transmissive regions than heatableregions. Thus, preferably, for a translational frequency of f, a smallerdeviation of the actual frequency from the approximate averagemodulation frequency x·f can be obtained, since a plurality of secondpositions at which the intensity is maximum can preferably be passed atsubstantially the same rate.

In a preferred embodiment of the modulatable infrared emitter, theactuator is a MEMS actuator, preferably selected from the groupcomprising electrostatic actuator, piezoelectric actuator,electromagnetic actuator, and/or thermal actuator.

A MEMS actuator is preferably an actuator that is manufactured usingstandard microsystems technology manufacturing methods and alsoadvantageously exhibits dimensions in the order of μm. Such an actuatoris particularly compact, robust and low-maintenance and can bemanufactured easily and inexpensively. In particular, a number of partsof the emitter can be MEMS elements, i.e., elements with the preferredproperties mentioned above, and can be manufacturable in onemanufacturing step with the MEMS actuator. Desirably, the same substratecan be used in parts for fabrication. This simplifies and cheapens themanufacturing process.

The above actuators are particularly well suited for a large number offast, periodic translational movements and have low energy requirements,especially due to their compact design. The range of achievabletranslation speeds is moreover high due to the compact design, lowinertias and linear motion.

For modulation purposes, it may be desirable to dissipate heat from themicro-heating element to the largest possible extent preferably throughthe coupled actuator to the housing. Therefore, it may be preferred thatthe actuator is substantially or partially made of the housing material.

In further preferred embodiment of the modulatable infrared emitter, theMEMS actuator is an electrostatic actuator in the form of a comb drivebased on a variation in comb overlap and/or comb spacing.

MEMS comb drives are known from the prior art, e.g. from patentapplication DE 10 2017 206 183 A1. Depending on the embodiment, the comboverlap and/or the comb spacing can be varied.

It has been recognized that such MEMS comb drives due to theirdimensions and generatable movements are particularly suitable for apreferential translational motion and compact IR emitter.

In a further preferred embodiment of the modulatable infrared emitter,the non-transmissive regions of the aperture structure have atransmittance of less than 0.1 in a wavelength range within 780 nm to 1mm and the transmissive regions of the aperture structure have atransmittance of greater than 0.9.

Preferably, the transmittance describes the portion of the intensity ofthe IR radiation generated by the micro-heating element incident on therespective region that completely traverses (penetrates) the region. Thetransmittance depends on the spectrum of the incident radiation, thematerial used and the thickness of the material to be traversed. Aperson skilled in the art knows how to achieve the desired properties.It is particularly preferred that the non-transmissive (opaque) regionshave a transmittance of less than 0.05, more preferably 0.01 andespecially less than 0.005.

The transmissive regions preferably have a transmittance of more than0.95 and especially of 0.99.

As described above, it is preferred that the non-transmissive regionsare essentially reflective and only weakly absorptive so that theaperture structure does not heat up excessively and emit IR radiationitself.

The aperture structure preferably comprises metals, in particular metalsselected from the group comprising aluminum (Al), copper (Cu), gold(Au), silver (Ag), dielectric material such as Al(MgF₂) and/oralternating layers. In particular, these materials can be used either assolid material (plate) with a thickness of preferably >1 μm and/or ascoating of a thickness of typically 100 nm-1 μm. Particularlypreferably, the aforementioned materials and/or layer thicknesses cancreate non-transmissive regions in the aperture structure which preventunwanted IR emission with high efficiency.

With an aperture structure designed in this manner, the preferredemission and modulation characteristics of the emitter with a desiredextinction ratio can be achieved in conjunction with a geometricmatching between the regions of the aperture structure and the regionsof the micro-heating element.

In another aspect, the invention relates to a manufacturing method foran infrared emitter as described above, wherein the manufacturing of themicro-heating element comprises the following steps:

-   -   etching of the substrate;    -   deposition of a conductive material on the substrate;    -   optionally, patterning (structuring) the conductive material to        form a heatable layer;    -   contacting the conductive material.

For example, one of the preferred materials mentioned above can be usedas the substrate. During etching, a blank, for example a wafer, can beformed into the desired basic shape of the micro-heating element. In anext step, the conductive material for the heatable layer is deposited.In particular, the heatable regions are to be included.

If further structuring (patterning) of the conductive material isdesired, this can be carried out, for example, by further etchingprocesses. Likewise, additional material can be deposited or doping canbe carried out by usual processes.

For contacting the conductive material, suitable material such ascopper, gold and/or platinum can additionally be deposited on theconductive material by common processes. Physical vapor deposition(PVD), chemical vapor deposition (CVD) or electrochemical deposition canpreferably be used to this end.

In this way, a particularly finely structured micro-heating element canbe produced, which preferably has dimensions in the micrometer range.Likewise, these manufacturing steps have proven particularly successfuland belong to standard process steps in semiconductor processing.

In a further preferred embodiment of the manufacturing process, etchingand/or patterning (structuring) is selected from the group comprisingdry etching, wet chemical etching and/or plasma etching, in particularreactive ion etching, reactive ion deep etching (Bosch process); and/ordeposition selected from the group comprising physical vapor deposition(PVD), in particular thermal evaporation, laser beam evaporation, arcevaporation, molecular beam epitaxy, sputtering, chemical vapordeposition (CVD) and/or atomic layer deposition (ALD).

These processes are particularly suitable for the fabrication of finestructures with sizes in the micrometer range. In particular, the Boschprocess can produce very fine structures with a high aspect ratio, whichare advantageous for a compact, efficient micro-heating element that ispreferably fully integrated into the rest of the emitter structure.

In another aspect, the invention relates to a system comprising

a) a modulatable infrared emitter described herein

b) a control device,

wherein the control device is configured to regulate the actuator forrelative movement of the heating element and the aperture structurebetween a first position and a second position.

The control device preferably enables an input and converts this inputinto suitable control signals. For example, an input may be a desiredspectrum, intensity, and/or modulation frequency. The control deviceprimarily generates appropriate analog electrical signals, which arepassed to the actuator and/or the micro-heating element to generate thedesired IR radiation.

However, more complex signals can also serve as input, which specify anexact temporal amplitude curve of the outgoing IR radiation for adesired spectrum. The control device in this case also preferablyprovides suitable control signals for generating the desired modulatedIR radiation.

In particular, the control device is configured for a regulation of theactuator for the relative movement between heating element and aperturestructure between (at least) a first and (at least) a second position.For this purpose, electrical signals are generated which trigger therequired translational movement of the actuator.

Preferably, the control device comprises a control loop, wherein afeedback mechanism can be used to correct a discrepancy between desiredcontrol and actual movement of the actuator and/or heating of themicro-heating element.

It may be preferred that also the temperature profile of themicro-heating element for additional slow modulation of the IR radiationcan be regulated by the control device.

The control device of the system can be positioned externally orintegrated into the IR emitter.

The control device preferably comprises a processor, for example amicroprocessor. Other integrated circuits used in digital electronicsfor control may likewise be used.

The use of such a system include a suitable control device canconsiderably simplify the desired use of the IR emitter. For example,suitable spectroscopy signals can be designed on a PC. Via the input,the desired signals are subsequently transmitted to the control device.The control device in turn generates the drive signals, which produces acorresponding IR signal in high agreement with the theoreticalspecifications.

A control device, in particular in the form of a controller integratedin the emitter, is very compact and easy to handle. The control devicepreferably comprises a suitable interface for connection to a computer,for example. It may also be desirable that data can be transferred fromthe controller to the input device via this interface, such as thecurrent temperature of the heating element or other status information.

In a further preferred embodiment of the system, the control device isconfigured to regulate the temperature of the heatable regions of themicro-heating element, preferably in a range between 50° C. and 1000° C.

Such a control device is preferably capable of providing suitableelectrical power to the micro-heating element. In particular, it shouldbe possible to adjust the temperature sufficiently precisely and/or tokeep the temperature constant. A control mechanism with a feedback loopcan be used to this end. To measure the current temperature of themicro-heating element, for example, at least one temperature sensor canbe integrated at a suitable location on the heating element.

Such a control device allows the spectrum and/or the intensity of the IRemitter to be controlled particularly easily and reliably.

In a further preferred embodiment of the system, the control device isconfigured to regulate the actuator for oscillatory relative movement ofthe heating element and the aperture structure, passing (traversing) atleast a first and second position during a period of the oscillation.

Preferably, the translational movement triggered by the actuator isrepeated regularly between (at least) a first and (at least) a secondposition, so that an oscillation occurs between the positions and thetranslational movement exhibits a periodicity. Thereby, at the end ofthe translational movement, the starting point of the movement shallpreferably to be reached again and the movement is to be executed anewin the following period. At a translation frequency of f, as mentionedabove, the number of passes (traverses) x of a first and a secondposition indicates the resulting modulation frequency by x·f.Preferably, the same first and/or second positions can be passed severaltimes as well as several first and/or second positions can be passed onetranslation period.

It is also possible to make a stepless adjustment of the translationfrequency and thus the modulation frequency within the scope of theelectronic resolution and/or bandwidth of the control device and/or theactuator. Thus, the modulation frequency can preferably be varied overtime.

It may be further preferred that not only the translation frequency butalso the translation amplitude is varied within the range of motionpossibilities of the actuator. For example, depending on the design ofthe aperture structure and/or the heating element, the number ofdifferent first and/or second positions passed (traversed) within onetranslation period can be varied. Thus, for example, as described above,the modulation frequency of the IR radiation can also be varied whilethe translation frequency remains constant.

Thereby a system is provided through which a very flexible and efficientvariation of the modulation frequency of the IR radiation can beachieved.

In a further preferred embodiment of the system, the control device isconfigured to regulate the actuator for an oscillatory relative movementof the heating element and the aperture structure such that a modulationfrequency of the radiant power of the emitted infrared radiation isachieved between 10 Hz and 100 kHz, particularly preferably between 100Hz and 20 kHz.

To this end it is particularly preferred that all required components,such as control equipment, actuator, etc., enable the requiredbandwidth.

The above frequencies have proven to be particularly effective for thepreferred applications in the field of spectroscopy. In particular,these frequencies have proven to be especially suitable for use inphotoacoustic spectroscopy, as they cover a wide range of acousticfrequencies, the generation of which is the primary focus of thisspectroscopy method.

In another aspect, the invention relates to a method for modulatedemission of infrared radiation comprising.

-   -   providing a modulatable infrared emitter according to any of the        described embodiments;    -   heating the heatable regions of the micro-heating element to        emit an infrared radiation;    -   controlling the actuator for relative movement of the aperture        structure and the micro-heating element between at least a first        position and a second position to modulate the radiant power of        the emitted infrared radiation.

The average person skilled in the art will recognize that technicalfeatures, definitions and advantages of preferred embodiments of the IRemitter and system according to the invention also apply to the methodaccording to the invention.

In another aspect, the invention relates to the use of a modulatableinfrared emitter according to the preceding description or a systemaccording to the preceding description for a spectroscopy methodselected from the group comprising photoacoustic spectroscopy and/orinfrared spectroscopy.

The described IR emitter may especially be used in infraredspectroscopy. However, a compact, long-life IR emitter that has a broadspectrum and can be modulated is of interest for a variety ofapplications.

For example, time-resolved measurements can be used to select specificfrequency ranges of the IR emitter by using a tunable filter to selectdifferent frequencies of the spectrum of the emitter at different times.Modulation can in turn block certain frequencies from this and transmitothers, so that IR pulses with essentially well-defined frequencies areemitted. As a result, in a time-resolved recording, e.g. of anabsorption spectrum, the frequency absorbed in each case can bedetermined precisely.

The use of a compact, long-life and high-frequency modulatable IRemitter in photoacoustic spectroscopy is particular advantageous.Especially for photoacoustic spectroscopy, many applications areconceivable that do not take place in the laboratory and must functionin everyday life. Examples are military applications for the detectionof poisonous gas or the detection of (harmful) substances in the ambientair. Due to the high modulation frequencies, better signal-to-noiseratios can be achieved compared to direct modulation of the heatingelement, and a non-direct modulated emitter is also more durable.

In another aspect, the invention relates to a photoacoustic spectroscopefor analyzing gas, comprising.

-   -   a modulatable infrared emitter according to any of the foregoing        described embodiments,    -   an analysis volume fillable with gas,    -   an acoustic detector,        wherein the analysis volume is positioned between the infrared        emitter and the acoustic detector so that the infrared radiation        modulatably emitted by the infrared emitter can be used for        photoacoustic spectroscopy of the gas.

The person skilled in the art is familiar with photoacousticspectroscopy, how the technique is carried out and which components areused in the process. Due to the compact and long-life IR emitter, whichis not known from the prior art, the whole setup can be manufactured ina particularly compact way suitable for everyday use. Due to the highmodulation frequencies, the analysis possibilities are extremelyversatile. At the same time, the signal-to-noise ratio can be increased,which is better for an acoustic detector with higher frequencies. Atypical 1/f noise can thus be reduced, for example.

DETAILED DESCRIPTION

In the following, the invention will be explained in more detail bymeans of examples and figures, without being limited to them.

SHORT DESCRIPTION OF THE IMAGES

FIG. 1 shows a schematic diagram of the IR emitter.

FIG. 2 shows a schematic representation of the IR emitter during atranslation period of the heating element to modulate the IR beam attime T=0.

FIG. 3 shows a schematic representation of the IR emitter during atranslation period of the heating element to modulate the IR beam attime T=¼.

FIG. 4 shows a schematic representation of the IR emitter during atranslation period of the heating element to modulate the IR beam attime T= 2/4.

FIG. 5 shows a schematic representation of the IR emitter during atranslation period of the heating element to modulate the IR beam attime T=¾.

FIG. 6 shows a schematic representation of the IR emitter during atranslation period of the heating element to modulate the IR beam attime T=1.

DETAILED DESCRIPTION OF THE IMAGE

FIG. 1 shows a schematic cross-sectional view of the modulatableinfrared emitter 1. The IR emitter is accommodated in a housing 18,which consists of a lower support 19, side parts 23 and a cover element21. Sealing elements 25 may be present between the support 19, coverelement 21 and side parts 23, respectively. These sealing elements 21are used to reduce thermal exchange of the interior of the emitter 1, inwhich the micro-heating element 5 is present, with the externalenvironment of the IR emitter 1. The cover element 21 comprises anapplied aperture structure 3 at the top. The structured micro-heatingelement 5 within the housing 18 comprises individual, parallel heatinglamellae 17. The surfaces of the heating lamellae 17 oriented in thedirection of the aperture structure 3 represent heatable regions 9 in afirst plane 10. Along said first plane 10, periodically arrangednon-heatable regions 11 are located between the periodically arrangedheatable regions 9. The aperture structure is arranged along a secondplane 12, which is parallel to the first plane 10 and consists ofregions 13 which are transmissive (transparent) to infrared radiationand regions 15 which are non-transmissive (opaque). These are alsoarranged periodically and have the same period. The relative movementbetween the heating element 5 and the aperture structure 3 is realizedby an actuator 7 in the form of a comb drive, which is directly coupledto the micro-heating element 5. The actuator 7 is in turn attached to aside part 23 of the housing 18. The micro-heating element 5 isfree-standing except for the connection to the actuator 7.

The number of non-transmissive regions 15 of the aperture structure 3 isequal to the number of heatable regions 9 of the micro-heating element5. The width of the non-transmissive regions 15 is slightly wider thanthat of the heatable regions 9 so that their IR radiation issubstantially blocked when the heatable regions 9 are positioned in afirst position directly below the non-transmissive regions 15 by meansof the actuator 7. In said first position, the radiation emitted fromthe IR emitter 1 exhibits a minimum intensity. By moving the heatableregions 9 to a second position (not shown) below the transmissiveregions 13 of the aperture structure 3, a maximum intensity of theemitted beam can be set. In this case, the regions are designed in sucha way that an extinction ratio between the intensity of the radiationemitted in the first position and the intensity of the radiation emittedin the second position of at least 2 is achieved.

FIG. 2 shows the modulatable infrared emitter 1 of FIG. 1 during atranslation period, at time T=0, at the beginning of the period. Here,all heatable regions 9 of the micro-heating element 5, which is directlycoupled to the actuator 7, are positioned by the latter in a firstposition directly below the non-transmissive (opaque) regions 15 of theaperture structure 3. In this case, the unmodulated radiation 29 emittedby the heatable regions 9 is substantially absorbed and/or reflected bythe non-transmissive regions 15 and the emitted intensity of the IR beamis minimal. In the embodiment shown, a lens 27 is present on the emitterabove the aperture structure 3 and used to collimate the modulatedinfrared beam.

FIG. 3 shows the modulatable infrared emitter 1 during the translationperiod at time T=¼, after one quarter of the period length. Here, allheatable regions 9 of the micro-heating element 5 are positioned by theactuator 7 in a second position directly below the transmissive regions13 of the aperture structure 3. The translational movement of themicro-heating element 5 by the actuator 7 proceeds to the right.Thereby, the unmodulated radiation 29 essentially radiates through thetransmissive regions 13 and the emitted intensity of the IR beam ismaximal.

FIG. 4 is a representation of the modulatable infrared emitter 1 duringthe translation period at time T= 2/4, after half of the full periodduration. The micro-heating element 5 has been translated back to theinitial position to the left. As at time T=0 in FIG. 2 , all heatableregions 9 of the micro-heating element 5 are positioned by the actuator7 in the (same) first position directly below the non-transmissiveregions 15 of the aperture structure 3 and the unmodulated radiation 29is substantially absorbed and/or reflected. The emitted intensity of theIR beam is again minimal.

FIG. 5 shows the modulatable infrared emitter 1 during the translationperiod at time T=¾, after three quarters of the period length haspassed. The heatable regions 9 of the micro-heating element 5 have beentranslated further to the left by the actuator 7 to another secondposition directly below the transmissive regions 13 of the aperturestructure 3. The unmodulated radiation 29 now again radiates essentiallythrough the transmissive regions 13, and the emitted intensity of the IRbeam is again at a maximum.

In FIG. 6 , at the end of the translation period, the modulatableinfrared emitter 1 has translated back to the right, to the startingpoint of the movement. The heatable regions 9 are again in the firstposition, just below the non-transmissive regions 15. The unmodulatedradiation 29 is essentially absorbed and/or reflected and the intensityof the IR beam is minimal. Now a new translation period can start anewwith the same sequence. The end time of the shown period coincides withthe start time of the following period.

In a traversed translation period, as shown in FIGS. 3-6 , the firstposition was passed twice and two different second positions passedonce. The end point of the period is assigned to the next period, whosestarting point it represents. Thus, the intensity was twice minimum andmaximum within one translation period. At a translation frequency of f,the IR beam is thus modulated with an average frequency of about 2·f.

It is noted that various alternatives to the described embodiments ofthe invention may be used to carry out the invention and arrive at thesolution according to the invention. Thus, the infrared emitteraccording to the invention, the system, and methods and uses thereof arenot limited in their embodiments to the foregoing preferred embodiments.Rather, a multitude of embodiments is conceivable, which may deviatefrom the solution presented. The aim of the claims is to define thescope of protection of the invention. The scope of protection of theclaims is directed to covering the infrared emitter according to theinvention, the system, methods of their use as well as equivalentembodiments thereof.

LIST OF REFERENCE SIGNS

-   1 modulating infrared emitter-   3 aperture structure-   5 structured micro-heating element-   7 actuator-   9 heatable regions-   10 first state-   11 non-heatable regions-   12 second state-   13 transmissive (transparent) regions-   15 non-transmissive (opaque) regions-   17 heating lamella-   18 housing-   19 support-   21 cover element-   23 side parts-   25 sealing elements-   27 lens-   29 unmodulated radiation

The invention claimed is:
 1. A modulatable infrared emitter comprising an aperture structure, a structured micro-heating element and an actuator, wherein the micro-heating element exhibits in a first plane heatable and non-heatable regions, the aperture structure exhibits in a second plane transmissive regions and non-transmissive regions for infrared radiation, the two planes being parallel to one another, the aperture structure and the micro-heating element are movable in the parallel planes relative to each other, and the actuator is configured for a relative movement of the aperture structure and the micro-heating element between at least a first and a second position, such that an extinction ratio of at least 2 is achievable for the infrared radiation emittable by the micro-heating element through the aperture structure between the first and second position, wherein in the first position the IR radiation emittable by the heatable regions is predominantly absorbed and/or reflected by the non-transmissive regions of the aperture structure, while in the second position the IR radiation emittable by the heatable regions predominantly radiates through the transmissive regions of the aperture structure.
 2. The modulatable infrared emitter according to claim 1, wherein the actuator is coupled to the heating element and is configured for translational movement of the heating element relative to the aperture structure, or the actuator is coupled to the aperture structure and is configured for translational movement of the aperture structure relative to the heating element.
 3. The modulatable infrared emitter according to claim 1, wherein the infrared emitter comprises a housing in which the aperture structure, the micro-heating element and the actuator are installed.
 4. The modulatable infrared emitter according to claim 1, wherein the micro-heating element comprises a substrate on which at least partially a heatable layer of a conductive material is deposited, on which contacts for a current and/or voltage source are present.
 5. The modulatable infrared emitter according to claim 1, wherein the micro-heating element comprises a lamellar structure, a meander structure and/or a grid structure.
 6. The modulatable infrared emitter according to claim 1, wherein the actuator is a MEMS actuator.
 7. The modulatable infrared emitter according to claim 1, wherein the non-transmissive regions of the aperture structure exhibit a transmittance of less than 0.1 in a wavelength range within 780 nm to 1 mm and the transmissive regions of the aperture structure exhibit a transmittance of more than 0.9.
 8. The modulatable infrared emitter according to claim 1, wherein the infrared emitter comprises a housing in which the aperture structure, the micro-heating element and the actuator are present installed, wherein the aperture structure is thermally decoupled from the housing or wherein the housing comprises a cover element in which the aperture structure is present fixated and at least one optical filter is additionally installed in the cover element.
 9. The modulatable infrared emitter according to claim 1, wherein with respect to the possible relative movement between the micro-heating element and the aperture structure, in the first plane the heatable regions and non-heatable regions of the micro-heating element and in the second plane the transmissive regions and non-transmissive regions of the aperture structure are arranged periodically.
 10. The modulatable infrared emitter according to claim 1, the actuator is a MEMS actuator selected from the group comprising electrostatic actuator, piezoelectric actuator, electromagnetic actuator.
 11. The modulatable infrared emitter according to claim 1, the actuator is an electrostatic MEMS actuator in the form of a comb drive based on a variation of the comb overlap and/or the comb spacing.
 12. A manufacturing method for an infrared emitter according to claim 1, wherein the manufacture of the micro-heating element comprises the following steps: etching of the substrate; deposition of a conductive material on the substrate; optionally, patterning the conductive material to form a heatable layer; and contacting the conductive material.
 13. The manufacturing method according to claim 12, wherein etching and/or patterning is selected from the group consisting of dry etching, wet chemical etching, plasma etching, reactive ion etching, and reactive ion deep etching (Bosch process); or the deposition is selected from the group consisting of physical vapor deposition (PVD), thermal evaporation, laser beam evaporation, arc evaporation, molecular beam epitaxy, sputtering, chemical vapor deposition (CVD) and atomic layer deposition (ALD).
 14. A system comprising: a) a modulatable infrared emitter according to claim 1, and b) a control device, wherein the control device is configured for regulating the actuator for relative movement of the heating element and the aperture structure between a first and a second position.
 15. The system according to claim 14, wherein the control device is configured to regulate the temperature of the heatable regions of the micro-heating element.
 16. The system according to claim 14 wherein the control device is configured to regulate the actuator for an oscillating relative movement of the heating element and the aperture structure, wherein during a period of the oscillation at least a first and a second position are passed.
 17. The system according to claim 14 wherein the control device is configured to regulate the actuator for an oscillating relative movement of the heating element and the aperture structure such that a modulation frequency of the radiant power of the emitted infrared radiation between 10 Hz and 100 kHz is achieved.
 18. A method for a modulated emission of infrared radiation comprising: providing a modulatable infrared emitter according to claim 1; heating the heatable regions of the micro-heating element to emit an infrared radiation; and controlling the actuator for relative movement of the aperture structure and the micro-heating element between at least a first position and a second position to modulate the radiant power of the emitted infrared radiation.
 19. A method of performing photoacoustic spectroscopy and/or infrared spectroscopy comprising using a modulatable infrared emitter according to claim
 1. 20. A photoacoustic spectroscope for the analysis of gas, comprising: a modulatable infrared emitter according to claim 1, an analysis volume fillable with gas, and an acoustic detector, wherein the analysis volume is positioned between the infrared emitter and the acoustic detector so that the infrared radiation modulatably emitted by the infrared emitter can be used for photoacoustic spectroscopy of the gas. 